1. Field
The present disclosure relates to a solid-state image pickup apparatus of a Charge Coupled Device (CCD) type which is used as an image taking section of, for example, video cameras and digital still cameras, and a fabrication method for the same, and in particular, to a solid-state image pickup apparatus of a CCD type which is intended to have a higher image quality and a smaller size, and a fabrication method for the same.
2. Description of the Related Art
In recent years, solid-state image pickup apparatuses of CCD type are widely used instead of image pickup tubes since they have characteristics that they are lighter and have longer lives compared to the image pickup tubes. Particularly, recently, such solid-state image pickup apparatuses of CCD type are widely used not only in video cameras for household use, video cameras for broadcasting and digital still cameras, but also in cell-phones, various household electric appliances which are referred to as digital electric appliances, and the like.
FIG. 14 is a cross-sectional view showing an exemplary basic structure of one pixel of a conventional solid-state image pickup apparatus.
As shown in FIG. 14, a solid-state image pickup apparatus 100 includes: a photodiode 102 formed of an n-type diffusion layer which is provided on a p-type S1 substrate 101 as a semiconductor substrate: a vertical CCD channel 103 formed of an n-type region which is provided adjacent to the photodiode 102 on one side; and an overflow drain 104 provided on the other side of the photodiode 102. Above a region between the photodiode 102 and the vertical CCD channel 103, a transfer electrode 106 which also serves as a readout electrode is provided via a gate insulation film 105. Above a region between the photodiode 102 and the overflow drain 104, an overflow potential control electrode 107 is provided via the gate insulation film 105. A planarizing insulation film 108 is provided to cover the substrate. Thereon, a light shielding film 109 which has an opening above the photodiode 102 is further provided.
In the above-described structure, incident light hr passes through the opening of the light-shielding film 109 and enters the photodiode 102, thereby photoelectric converted, and stored as a signal charge. The signal charge is transported from the photodiode 102 to the vertical CCD channel 103 through a field shift gate 110 under the transfer electrode 106, and is transferred within the vertical CCD channel 103 sequentially. An excess charge caused by strong incident light hr is discharged towards the overflow drain 104 via an overflow gate 111 under the overflow potential control electrode 107.
FIG. 15 is a cross-sectional view showing an exemplary structure of one pixel of another conventional solid-state image pickup apparatus. In FIG. 15, components having the same functions and effects as those in FIG. 14 are denoted by the same reference numerals.
As shown in FIG. 15, a solid-state image pickup apparatus 120 includes a p-type well diffusion layer loin provided on an n-type S1 substrate 101A which is thin under a photodiode 102 and thick under a vertical CCD channel 103. An excess charge caused by strong incident light hr is to be discharged through the well diffusion layer 101B to the substrate 101A. Such a structure to discharge an excess charge is called a vertical overflow drain structure.
FIG. 16 is a diagram showing a potential in X-Y direction of FIG. 14.
In FIG. 16, the horizontal axis indicates positions in X-Y direction, and the vertical axis indicates potential. The solid line represents a potential when a signal is stored and the broken line represents a potential when a signal is readout. The height of arrow A represents a potential difference between a potential at the field shift gate 110 during reading out and a potential at the overflow gate 111.
An excess charge when a signal is charged, represented by the solid line, is to be discharged towards the overflow drain 104 over a potential at the overflow gate 111.
FIG. 17 is a graph showing an input/output characteristic of the pixel portion of FIG. 14. In FIG. 17, the horizontal axis indicates incident light intensity, and the vertical axis indicates output current.
As shown in FIG. 17, when the incident light is weak, an input/output characteristic curve of an output current with respect to the incident light intensity follows a straight line 130 initially, and then the input/output characteristic curve bends around as shown by a solid line 132 when it exceeds a certain incident light intensity 131. The output current is suppressed and compressed.
This is because a part of the charges caused by the incident light exceeds the potential at the overflow gate 111 and is discharged toward the overflow drain 104. A range of the straight line 130 in which the input/output characteristic curve is linear is decided by potential difference A between the potential of reading out of the field shift gate 110 and the potential of overflow gate 111.
However, since the field shift gate 110 and the overflow gate 111 are completely different from each other, potential differences A are different for each of the pixels. Thus, the range of the straight line 130 is small in a pixel having a small potential difference A, and the input/output characteristic curve becomes the curve represented as 132A in FIG. 17. On the other hand, the range of the straight line 130 is large in a pixel having a large potential difference A, and the input/output characteristic curve becomes the curve represented as 132B in FIG. 17. With such a variance in ranges, display unevenness occurs on a reproduced screen. Thus, they cannot be used. Accordingly, a range that can be used is limited to the range indicated by arrow B in FIG. 17 (a range of the straight line 130 to the input/output characteristic curve 132A), and a dynamic range becomes small.
As described above, the conventional structure has a problem that display unevenness occurs on a reproduced screen since there is a potential difference A between the field shift gate 110 and the overflow gate 111 for each of the pixels.
As a means for solving the above-described problem, for example, Japanese Laid-Open Publication No. 05-137072 discloses a solid-state image pickup apparatus having a structure as shown in FIG. 18.
FIG. 18 is a cross-sectional view showing an exemplary structure of one pixel of yet another conventional solid-state image pickup apparatus. In FIG. 18, components having the same functions and effects as those in FIGS. 14 and 15 are denoted by the same reference numerals.
As shown in FIG. 18, a solid-state image pickup apparatus 140 includes an overflow drain 104 provided on a side of a vertical CCD channel 103 which is opposite to a side of a photodiode 102, from which a signal charge is to be read out from incident light hr, unlike the conventional solid-state image pickup apparatus 100 shown in FIG. 14.
In such a solid-state image pickup apparatus 140, the incident light h which passes through an opening 109a of the light-shielding film 109 is photoelectric converted by the photodiode 102 and is stored as a signal charge. The signal charge passes through a field shift gate 110 under a transfer electrode 106, transported from the photodiode 102 to a vertical CCD channel 103, and transferred within the vertical CCD channel 103 sequentially.
An excess charge caused by strong incident light hr is discharged from the vertical CCD channel 103 towards an overflow drain 104 through the field shift gate 110, an embedded channel of the vertical CCD channel 103, and an overflow gate 111.
In such a structure, the incident light intensity 131 where the input/output curve bends around as shown in FIG. 17 is determined based on only a potential at the field shift gate 111 when a charge is stored. Thus, a saturation characteristic can be evened substantially. Accordingly, output unevenness among pixels due to discharging an excess charge can be made smaller, and a dynamic range can be expanded.
However, in any of the conventional structures a s described above, the photodiode 102 for photoelectric converting the incident light hr and generating a signal charge, and the transfer electrode 106 and the vertical CCD channel 103 for transferring the generated signal charge are arranged on a two-dimensional plane on the semiconductor substrate. Thus, larger light receiving area cannot be secured. Accordingly, it is difficult to aim at improving image quality by photoelectric converting more incident light hr.
As another method for improving image quality, increasing an amount of a signal charge which can be stored in the vertical CCD channel 103 for storing the signal charge which is photoelectric converted by the photodiode 102. This becomes possible by increasing an area of the vertical CCD channel 103 and a voltage to be applied to the transfer electrode 106 of the vertical CCD channel 103. However, when the area of the vertical CCD channel 103 is increased, the area occupied by the vertical CCD channel 103 on the semiconductor substrate becomes large, and thus, the light receiving area of the photodiode 102 has to be reduced. Accordingly, the light intensity of the incident light hr for photoelectric conversion at the photodiode 102 becomes small, and it becomes difficult to aim at improving the image quality. When the voltage is applied to the transfer electrode of the vertical CCD channel 103, the operating voltage in the semiconductor device becomes large. This is not preferable in terms of power consumption and miniaturization of solid-state image pickup apparatuses.
As described above, with the structures of the conventional solid-state image pickup apparatuses 100, 120, 140 in which the photodiode 102, the transfer electrode 106 and the vertical CCD channel 103 are arranged on a two-dimensional plane on the semiconductor substrate it is difficult to increase the light intensity of the incident light hr to the photodiode 102, and to increase a storage amount for the signal charge which is photoelectric converted. Thus, it is difficult to aim at improving the image quality of the solid-state image pickup apparatus 100, 120, and 140.